U.S. patent application number 15/320721 was filed with the patent office on 2017-05-18 for optical element comprising a reflective coating.
The applicant listed for this patent is Fraunhofer-Gesellschaft zur Forderung der angewandten Forschung e.V., Friedrich-Schiller-Universitat Jena. Invention is credited to Norbert KAISER, Mark SCHURMANN, Stefan SCHWINDE.
Application Number | 20170139085 15/320721 |
Document ID | / |
Family ID | 53398088 |
Filed Date | 2017-05-18 |
United States Patent
Application |
20170139085 |
Kind Code |
A1 |
SCHURMANN; Mark ; et
al. |
May 18, 2017 |
Optical Element Comprising a Reflective Coating
Abstract
An optical element including a reflective coating is disclosed.
In an embodiment the reflective coating includes an
adhesion-promoting layer, an at least partially reflective silver
layer disposed on the adhesion-promoting layer and a protective
layer system disposed on the silver layer, wherein the protective
layer system includes a plurality of dielectric layers, wherein the
dielectric layers include at least one first layer and at least one
second layer, wherein the first layer and the second layer have a
different resistance to at least two different contamination
substances, wherein the dielectric layers have a thickness of not
more than 30 nm, and wherein a number of the dielectric layers
amounts to at least five.
Inventors: |
SCHURMANN; Mark; (Jena,
DE) ; SCHWINDE; Stefan; (Jena, DE) ; KAISER;
Norbert; (Jena, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fraunhofer-Gesellschaft zur Forderung der angewandten Forschung
e.V.
Friedrich-Schiller-Universitat Jena |
Muenchen
Jena |
|
DE
DE |
|
|
Family ID: |
53398088 |
Appl. No.: |
15/320721 |
Filed: |
June 12, 2015 |
PCT Filed: |
June 12, 2015 |
PCT NO: |
PCT/EP2015/063208 |
371 Date: |
December 20, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 5/0808 20130101;
G02B 1/14 20150115; G02B 27/142 20130101; G02B 5/0858 20130101 |
International
Class: |
G02B 1/14 20060101
G02B001/14; G02B 5/08 20060101 G02B005/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 20, 2014 |
DE |
10 2014 108 679.1 |
Claims
1-13. (canceled)
14. An optical element comprising a reflective coating, the
reflective coating comprising: an adhesion-promoting layer; an at
least partially reflective silver layer disposed on the
adhesion-promoting layer; and a protective layer system disposed on
the silver layer, wherein the protective layer system comprises a
plurality of dielectric layers, wherein the dielectric layers
comprise at least one first layer and at least one second layer,
wherein the first layer and the second layer have a different
resistance to at least two different contamination substances,
wherein the dielectric layers have a thickness of not more than 30
nm, and wherein a number of the dielectric layers amounts to at
least five.
15. The optical element according to claim 14, wherein each
dielectric layer of the protective layer system contains at least
one of the following materials: SiO.sub.2, Si.sub.3N.sub.4,
Al.sub.2O.sub.3, AlN, ZrO.sub.2, ZrN, HfO.sub.2, HfN, TiO.sub.2,
TiN, Ta.sub.2O.sub.5, TaN, Nb.sub.2O.sub.5, NbN, Y.sub.2O.sub.3,
YN, MgF.sub.2, MgO, LiF, AlF.sub.3.
16. The optical element according to claim 14, wherein the
contamination substances comprise chlorine, sulfur and/or compounds
of chlorine or sulfur.
17. The optical element according to claim 14, wherein the first
layer has a greater resistance to chlorine and/or chlorine
compounds than the second layer.
18. The optical element according to claim 14, wherein the first
layer is an aluminum oxide layer.
19. The optical element according to claim 14, wherein the second
layer has a greater resistance to sulfur and/or sulfur compounds
than the first layer.
20. The optical element according to claim 14, wherein the second
layer is a silicon oxide layer.
21. The optical element according to claim 14, wherein the
protective layer system contains at least two layer pairs made up
in each case of the first layer and the second layer.
22. The optical element according to claim 14, wherein the
protective layer system contains at least three layer pairs made up
in each case of the first layer and the second layer.
23. The optical element according to claim 14, wherein the
protective layer system is a reflection-enhancing interference
layer system which in an alternating manner has first layers with a
higher refractive index and second layers with a lower refractive
index.
24. The optical element according to claim 14, wherein the
protective layer system has at least one first layer stack with
alternating first layers and second layers and at least one second
layer stack with alternating first layers and second layers, and
wherein the first and second layer stacks have a differing
effective refractive index.
25. The optical element according to claim 14, wherein, in the
protective layer system, the at least one first layer and the at
least one second layer have layer stresses with an opposite
sign.
26. The optical element according to claim 14, wherein the
reflective coating is disposed on the optical element, the optical
element having a substrate with a curved surface.
27. An optical element comprising a reflective coating, the
reflective coating comprising: a adhesion-promoting layer; an at
least partially reflective metal layer disposed on the
adhesion-promoting layer; and a protective layer system disposed on
the reflective metal layer, the protective layer system containing
a plurality of dielectric layers, wherein the dielectric layers
comprise at least one first layer and at least one second layer,
wherein the first layer and the second layer have a different
resistance to at least two different contamination substances,
wherein the dielectric layers have a thickness of not more than 30
nm, wherein a number of the dielectric layers amounts to at least
five, wherein the first layer is an aluminum oxide layer, wherein
the second layer is a silicon oxide layer, and wherein the
protective layer system contains at least two layer pairs made up
in each case of the first layer and the second layer.
28. The optical element according to claim 27, wherein the
reflective metal layer is a silver layer.
Description
[0001] This patent application is a national phase filing under
section 371 of PCT/EP2015/063208, filed Jun. 12, 2015, which claims
the priority of German patent application 10 2014 108 679.1, filed
Jun. 20, 2014, each of which is incorporated herein by reference in
its entirety.
TECHNICAL FIELD
[0002] The invention relates to an optical element, in particular a
curved optical element, to which a reflective coating is
applied.
BACKGROUND
[0003] Numerous applications for precision-optics components
require a mirror layer which is highly reflective in the widest
possible spectral range. Metals such as, for example, gold,
aluminum or silver are therefore often used as a reflective
coating. Of the metals, silver has the highest reflectivity from
the visible to the infrared spectral range. However, silver is very
susceptible to corrosion. In order to avoid the corrosion of a
reflective metal layer, in particular a silver layer, a protective
layer can be applied to the metal layer.
[0004] It has been found, however, that even reflective silver
layers provided with a protective layer often do not have an
adequate long-term stability. If, for example, silver mirrors
provided with a protective layer are exposed for a relatively long
time to environmental conditions with very high air humidity, or if
condensation of such mirrors even occurs, liquid, in particular
with dissolved corrosive substances, can pass at these locations
through the protective layer to the corrosion-sensitive silver
layer. A corrosion process is initiated at these locations and then
propagates proceeding from this origin through the silver
layer.
SUMMARY OF THE INVENTION
[0005] Embodiments of the invention provide an improved optical
element comprising a reflective coating which is distinguished by
an improved resistance to contamination substances from the
surroundings.
[0006] According to at least one embodiment, the optical element
has a reflective coating, which is applied for example to the
surface of a substrate or to the surface of a, preferably polished,
layer applied to a substrate.
[0007] According to a preferred embodiment, the optical element has
a curved surface, the reflective coating being arranged on the
curved surface. By way of example, the curved surface can be the
surface of a substrate or the surface of a, preferably polished,
layer applied to a substrate. The optical element can be provided
for applications in precision optics, for example. The optical
element can be provided in particular for applications in astronomy
and/or for applications in space, for example for Earth
observation.
[0008] The reflective coating advantageously contains an
adhesion-promoting layer, which is applied for example as a first
layer to a substrate. During the production of the reflective
coating, the adhesion-promoting layer can be applied to the surface
of the optical element, for example, in a first step by a PVD
(Physical Vapor Deposition) process, such as, for example, thermal
evaporation, electron beam evaporation, plasma-enhanced
evaporation, magnetron sputtering or ion beam sputtering.
Alternatively, the adhesion-promoting layer can be applied by using
a CVD (Chemical Vapor Deposition) process or an ALD (Atomic Layer
Deposition) process. The adhesion-promoting layer has the function
in particular of improving the adhesion of a reflective metal layer
arranged thereabove. Furthermore, the adhesion-promoting layer can
simultaneously have the function of a diffusion barrier layer. In
its property as a diffusion barrier layer, the adhesion-promoting
layer can reduce in particular the diffusion between constituent
parts of a substrate of the optical element and a metal layer
following the adhesion-promoting layer, and vice versa.
[0009] The adhesion-promoting layer can be an individual layer or a
sequence of a plurality of layers. The adhesion-promoting layer is
advantageously formed from a material which ensures good adhesion
between the metal layer following it and the substrate. Suitable
materials for the adhesion-promoting layer are, for example, metals
such as Cr, Ti, Cu, Ru, Mo, W, semiconductors such as Si or SiC, or
dielectric layers such as SiO.sub.2, Si.sub.3N.sub.4,
Al.sub.2O.sub.3, AlN, ZrO.sub.2, ZrN, HfO.sub.2, HfN,
Ta.sub.2O.sub.5, TaN, Nb.sub.2O.sub.5, NbN, Y.sub.2O.sub.3, YN, or
mixtures of these materials. The adhesion-promoting layer
preferably has a thickness of between 5 nm and 1000 nm,
particularly preferably between 10 nm and 100 nm.
[0010] An at least partially reflective metal layer follows the
adhesion-promoting layer in the reflective coating. Depending on
its layer thickness, the at least partially reflective metal layer
can be a highly reflective metal layer for forming a reflector or
alternatively an only partially reflective metal layer, for example
for forming a beam splitter. The metal layer of the reflective
coating preferably comprises silver, gold or aluminum. Particular
preference is given to silver, since silver has a particularly high
reflection. Although silver per se is susceptible to corrosion, the
corrosion in the case of the optical element described herein is
counteracted particularly effectively by the protective layer
system.
[0011] In the reflective coating, the metal layer is followed by a
protective layer system, which serves in particular to protect the
metal layer from environmental influences. The protective layer
system contains a plurality of dielectric layers, the dielectric
layers preferably comprising at least one first layer and at least
one second layer, the first layer and the second layer
advantageously having a different resistance to at least two
different contamination substances. By way of example, the at least
one first layer can be distinguished by a high resistance to a
first contamination substance and the at least one second layer can
be distinguished by a high resistance to a second contamination
substance which differs from the first contamination substance.
[0012] The contamination substances can be in particular those
substances which can arise under the environmental conditions of
the optical element, for example during storage, transportation
and/or operation. In the case of an optical element provided for
use in space, these can be the terrestrial environmental conditions
which can arise, for example, during the production, the storage or
the transportation of the optical element. By way of example, the
contamination substances can be liquids with which the optical
element can come into contact, where salts from the surroundings
may be dissolved in the liquids, for example. The contamination
substances may be in particular hygroscopic particles which contain
salts. Even at room temperature and with an air humidity of less
than 80%, these hygroscopic particles can lead to the formation of
defects.
[0013] In the case of the protective layer system described herein,
the fact that the first layer and the second layer have a different
resistance to two different contamination substances means that a
higher resistance to such contamination substances is
advantageously achieved compared to that which could be achieved by
an individual layer consisting of the first material or the second
material.
[0014] The protective layer system contains a plurality of
dielectric layers and preferably consists exclusively of dielectric
layers. It is advantageous if the dielectric layers of the
protective layer system are very thin layers. The thickness of the
dielectric layers of the protective layer system is preferably not
more than 30 nm, particularly preferably not more than 20 nm. It
has been found in particular that, when thicker dielectric layers
are used, greater defects can arise through contamination particles
on account of the greater layer stresses which are caused as a
result. In layers with a thickness of not more than 30 nm or
preferably of not more than 20 nm, the layer stresses are
advantageously low compared to significantly thicker layers. When
comparatively thin layers are used, it is advantageously possible
for more layers to be stacked in the protective layer system than
in the case of thicker layers given a predefined overall layer
thickness of the protective layer system, this resulting for
example from optical requirements such as the transmission or the
absorption. This has the advantage that the protective layer system
has more interfaces, which can form a barrier to the propagation of
particles or defects. The number of the dielectric layers of the
protective layer system preferably amounts to at least three, at
least five or even at least ten.
[0015] It is furthermore advantageous if the thickness of the
dielectric layers is at least 1 nm, preferably at least 2 nm. The
dielectric layers of the protective layer system can have a
thickness, for example, of between 1 nm and 30 nm, preferably
between 2 nm and 20 nm.
[0016] Suitable materials for the protective layer system are in
particular oxide layers, nitride layers or fluoride layers. In
particular, the dielectric layers of the protective layer system
can contain or consist of at least one of the following materials:
SiO.sub.2, Si.sub.3N.sub.4, Al.sub.2O.sub.3, AlN, ZrO.sub.2, ZrN,
HfO.sub.2, HfN, TiO.sub.2, TiN, Ta.sub.2O.sub.5, TaN,
Nb.sub.2O.sub.5, NbN, Y.sub.2O.sub.3, YN, MgF.sub.2, MgO, LiF,
AlF.sub.3.
[0017] It has been found that, in the event of the accumulation of
contaminations on an optical element, water molecules can be bound
to the contaminations through a hygroscopic action of the
contaminations, as a result of which a small droplet is formed on
the protective layer system. Components such as ions from the
contamination particles may be dissolved in said droplet, such as
to form an electrolyte which, without suitable protection, could
reach the reflective metal layer and damage it. The contamination
substances which are formed in this way can comprise in particular
chlorine, sulfur and/or compounds of chlorine or sulfur. Salts such
as NaCl or Na.sub.2S in particular can have a damaging effect on
reflective metal layers, in particular silver. It is therefore
advantageous if the first layer and/or the second layer have a high
resistance to the contamination substances chlorine, sulfur and/or
compounds of chlorine or sulfur.
[0018] According to an advantageous embodiment, the first layer has
a greater resistance to chlorine and/or chlorine compounds than the
second layer. Furthermore, it is advantageous if the second layer
has a greater resistance to sulfur and/or sulfur compounds than the
first layer. The terms "first layer" and "second layer" do not
refer to the sequence of the layers in the reflective coating, but
rather serve merely for distinguishing between the layers which
have different resistances to the different contamination
substances. In the reflective coating, the first layer can
therefore be arranged above the second layer or alternatively the
second layer can be arranged above the first layer. Furthermore,
the first layer and the second layer can be arranged in multi-layer
systems, which each have a plurality of first layers and/or second
layers. The protective layer system preferably has at least three
layers composed of at least two different dielectric materials
which are distinguished by a different resistance to the at least
two different contamination substances.
[0019] According to an advantageous embodiment, the first layer
comprises an aluminum oxide, in particular Al.sub.2O.sub.3. It has
been found in particular that an aluminum oxide layer has a good
resistance to chlorine and/or a chlorine compound, in particular
NaCl. An aluminum oxide layer is furthermore distinguished by a
high transparency in a wide spectral range, which ranges from the
UV range into the IR range.
[0020] The second layer preferably comprises a silicon oxide such
as, for example, SiO.sub.2. It has been found that a silicon oxide
layer in particular has a good resistance to sulfur and/or sulfur
compounds such as, for example, Na.sub.2S.
[0021] It is therefore particularly advantageous if the protective
layer system has at least one first layer consisting of an aluminum
oxide and at least one second layer consisting of a silicon oxide.
This can have the effect in particular that the protective layer
system has a good resistance to chlorine and/or chlorine compounds
and at the same time a good resistance to sulfur and/or sulfur
compounds.
[0022] A particularly good resistance to the at least two
contamination substances can be achieved by virtue of the fact that
the layer system contains at least two layer pairs made up in each
case of the first layer and the second layer. It is particularly
preferable for the protective layer system to even contain at least
three, at least four or even at least five layer pairs made up in
each case of the first layer and the second layer. The alternating
arrangement of a plurality of first layers and second layers in the
protective layer system particularly effectively prevents the
penetration of contamination substances as far as the reflective
metal layer.
[0023] In one embodiment of the reflective coating, a further
adhesion-promoting layer is arranged between the metal layer and
the protective layer system. The further adhesion-promoting layer
advantageously comprises a material which ensures good adhesion of
the protective layer system on the reflective metal layer. The
further adhesion-promoting layer can comprise in particular a metal
such as Cr, Ti, Cu, Ru, Mo, W, a semiconductor material such as Si
or SiC, a dielectric material such as SiO.sub.2, Si.sub.3N.sub.4,
Al.sub.2O.sub.3, AlN, ZrO.sub.2, ZrN, HfO.sub.2, HfN,
Ta.sub.2O.sub.5, TaN, Nb.sub.2O.sub.5, NbN, Y.sub.2O.sub.3, YN, or
mixtures of these materials. The thickness of the further
adhesion-promoting layer is advantageously between 0.1 nm and 40
nm, preferably between 0.5 nm and 20 nm. In a similar manner to the
adhesion-promoting layer arranged between the substrate and the
metal layer, the further adhesion-promoting layer can also have the
function of a diffusion barrier, in addition to its function as an
adhesion-promoting layer. In the case of the further
adhesion-promoting layer, the adhesion-promoting layer reduces the
diffusion between the reflective metal layer and the protective
layer system.
[0024] In an advantageous embodiment, the protective layer system
has an optical function in addition to its function as a protective
layer for the reflective metal layer. The protective layer system
can be in particular a reflection-enhancing interference layer
system in which the dielectric layers in an alternating manner have
a relatively low and a relatively high refractive index. In
particular, the protective layer system can contain a sequence of a
plurality of dielectric layers, which in an alternating manner have
a low refractive index, in particular a refractive index
n1.ltoreq.1.6, and a high refractive index, in particular a
refractive index n2>1.6. Through such a reflection-enhancing
interference layer system, the reflection of the reflective coating
in a predefined spectral range can be increased in particular by a
suitable selection of the layer thicknesses, it being possible for
these to be determined by simulation programs known per se.
[0025] In a further advantageous embodiment, the protective layer
system has at least one first layer stack with alternating first
layers and second layers and at least one second layer stack with
alternating first layers and second layers, wherein the layer
stacks have a differing effective refractive index. In this
embodiment, the effective refractive index is understood to mean
the refractive index averaged over the respective layer stack. The
effective refractive index of the layer stack is an approximation
for the refractive index of the layer stack if the thickness of the
first layers and second layers is small compared to the wavelength
used. In this embodiment, each of the layer stacks has
approximately the function of an optical layer, the thickness of
which corresponds to the thickness of the layer stack and the
refractive index of which corresponds to the effective refractive
index of the layer stack.
[0026] The materials of the first layers in the first layer stack
and of the first layers in the second layer stack can differ from
one another. Similarly, it is possible for the materials of the
second layers in the first layer stack and of the second layers in
the second layer stack to differ from one another. Alternatively,
however, it is also possible for the same materials to be used for
the first layers and/or second layers in the first and the second
layer stack. In this embodiment, the first and/or second layers in
the layer stacks do not differ in terms of the material, but have
different thicknesses in the layer stacks.
[0027] In a further advantageous embodiment of the optical element,
in the protective layer system, the at least one first layer and
the at least one second layer have layer stresses with an opposite
sign. In this embodiment, the material of the first layer and of
the second layer is advantageously chosen in such a manner that the
layers generate layer stresses with opposite signs, such that the
layer stresses are reduced or even compensate for one another. By
way of example, the protective layer system can have first and
second dielectric layers, where the first layers generate tensile
stresses and the second layers generate compressive stresses, or
vice versa. In addition to its function as protection against
contamination substances and if appropriate its optical function,
the protective layer system can therefore also have a third
function for reducing stresses in the reflective coating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The invention will be explained in more detail hereinbelow
with reference to two exemplary embodiments in connection with
FIGS. 1 and 2.
[0029] In the drawing:
[0030] FIG. 1 shows a schematic illustration of a cross section
through an optical element according to a first exemplary
embodiment, and
[0031] FIG. 2 shows a schematic illustration of a cross section
through an optical element according to a second exemplary
embodiment.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0032] Identical component parts or component parts with the same
action are provided in each case with the same reference signs in
the figures. The component parts shown and the size ratios of the
component parts in relation to one another are not to be considered
as true to scale.
[0033] The optical element according to the first exemplary
embodiment which is shown schematically in cross section in FIG. 1
has a substrate 10, which is for example a glass substrate or a
polished metal substrate. The substrate 10 preferably has an rms
roughness of not more than 2 nm. The optical element may be in
particular a curved optical element, in which for example the
substrate 10 has a curved surface 11. The optical element can be
provided in particular for applications in astronomy. Owing to its
high long-term stability, the optical element described herein is
particularly suitable for applications in space.
[0034] In the exemplary embodiment, a layer system serving as a
mirror layer for use in the visual spectral range is applied to the
curved surface 11 of the substrate 10. An adhesion-promoting layer
1 comprising, for example, Al.sub.2O.sub.3 is applied to the
substrate 10 as the first layer. The adhesion-promoting layer 1 and
also the further subsequent layers of the layer system have been
applied by means of magnetron sputtering in the exemplary
embodiment. As an alternative, the layers can be applied by using a
PVD process such as, for example, thermal evaporation, electron
beam evaporation, plasma-enhanced vapor deposition or ion beam
sputtering, a CVD process or an ALD process.
[0035] In the exemplary embodiment, a reflective metal layer 2 is
applied to the adhesion-promoting layer 1, said metal layer being a
silver layer. During the production of the silver layer, the
deposition parameters are preferably chosen in such a manner as to
produce the smoothest possible silver layer in order to avoid
reflection losses through scattering. When the silver layer is
deposited by means of magnetron sputtering, it has proven to be
expedient, for example, to use argon as the sputtering gas, in
which case, for example, a gas flow of 15 sccm, a sputtering power
of 500 watts and a target-substrate spacing of more than 100 mm are
set.
[0036] To protect the silver layer against environmental
influences, a protective layer system 3 containing a plurality of
dielectric layers 31, 32 has been applied to the silver layer 2. In
the exemplary embodiment, the protective layer system 3 is built up
from alternating Al.sub.2O.sub.3 layers 31 and SiO.sub.2 layers 32.
In particular, the protective layer system 3, proceeding from the
silver layer 2, contains two layer pairs made up in each case of an
Al.sub.2O.sub.3 layer 31 and an SiO.sub.2 layer 32 and also an
additional Al.sub.2O.sub.3 layer 31 as the last layer.
[0037] The Al.sub.2O.sub.3 layers 31 can be applied, for example,
by a reactive sputtering process, with a metallic aluminum target
being used. As the sputtering gas, use can be made of argon with a
gas flow of, for example, 35 sccm, with so much oxygen additionally
being added as reactive gas that a stoichiometric Al.sub.2O.sub.3
layer 31 is deposited. The Al.sub.2O.sub.3 layer 31 advantageously
has no significant absorption losses in the spectral range of 450
nm to 5000 nm. During the sputtering process, the oxygen partial
pressure is advantageously monitored by means of a lambda
probe.
[0038] The SiO.sub.2 layers 32 are likewise applied, for example,
by a reactive sputtering process, with a pure silicon target being
used. As the sputtering gas, use can be made of argon with a gas
flow of, for example, 35 sccm, with so much oxygen additionally
being added as reactive gas that a stoichiometric SiO.sub.2 layer
is deposited. The SiO.sub.2 layer 32 advantageously has no
significant absorption losses in the visual spectral range. As is
the case during the production of the Al.sub.2O.sub.3 layer 31, the
oxygen partial pressure is advantageously monitored by means of a
lambda probe.
[0039] In an embodiment of the protective layer system 3, the
Al.sub.2O.sub.3 layers 31 and the SiO.sub.2 layers 32 can each have
a thickness of 10 nm. In this case, the overall thickness of the
protective layer system 3 is only approximately 50 nm. The
comparatively thin alternating first layers 31 and second layers 32
form an effective barrier to contaminations, in spite of their
small thicknesses. This is achieved in particular by virtue of the
fact that the thin dielectric layers 31, 32 which are resistant to
different contaminations are arranged in an alternating manner, and
therefore each form a plurality of barriers for different
contaminations. Furthermore, layer stresses which would otherwise
promote the propagation of defects are reduced by the small
thicknesses of the first layers 31 and second layers 32.
[0040] As an alternative to the exemplary embodiment described
here, it would also be possible to use larger layer thicknesses of
approximately 120 nm, in order, for example, to achieve an increase
in the reflection in a specific spectral range through optical
interference effects. It is possible, for example, to achieve
reflection values of more than 95% with the layer system produced
in this way in the entire spectral range from 450 nm to 5000
nm.
[0041] The reflective coating is distinguished in particular by a
good long-term stability. In the case of the reflective coating
produced in the manner described here, after a climatic test
lasting several days at a temperature of 49.degree. C. and a
relative air humidity of 95%, no layer defects occurred and the
optical properties of the reflective coating did not change.
[0042] Furthermore, the resistance of the reflective coating to
substances which can arise in the case of terrestrial environmental
conditions was tested. Given contamination of the reflective
coating with NaCl and Na.sub.2S, damage to the reflective silver
layer 2 was determined neither in the regions with contamination by
NaCl nor in the regions with contamination by Na.sub.2S.
[0043] In contrast thereto, a comparative test carried out on a
silver layer with an individual SiO.sub.2 protective layer showed,
after contamination with NaCl and Na.sub.2S, damage to the silver
in the regions in which contamination with NaCl was present.
Furthermore, in the case of a comparative test carried out on a
silver layer with an individual Al.sub.2O.sub.3 protective layer,
it was determined that, after contamination with NaCl and
Na.sub.2S, there was damage to the silver in the regions in which
contamination with Na.sub.2S was present.
[0044] The protective layer system 3 described here, which has a
sequence of alternating Al.sub.2O.sub.3 layers 31 and SiO.sub.2
layers 32, simultaneously achieves protection of the silver layer 2
against sulfur-containing contaminations and chlorine-containing
contaminations. It is advantageous in particular that the layer
system has at least two layer pairs made up of the dielectric
layers 31, 32 resistant to the different contaminations, since this
has the effect that a multiple barrier is formed against
penetration as far as the silver layer 2 for each of the
contaminations.
[0045] Furthermore, it is advantageous that the alternating
dielectric layers 31, 32 have in an alternating manner a high
refractive index and a low refractive index. For instance, the
refractive index of the Al.sub.2O.sub.3 layers 31 in the visible
spectral range is approximately 1.7. The refractive index of the
SiO.sub.2 layers 32 is approximately 1.46 in the visible spectral
range. In addition to its protective function, the protective layer
system 3 can therefore form an optical interference layer system in
order, for example, to increase the reflection of the reflective
coating in a specific spectral range. To achieve a reflection
maximum given a predefined wavelength, suitable layer thicknesses
for the dielectric layers 31, 32 can be determined by simulated
calculations.
[0046] The second exemplary embodiment of an optical element as
shown in FIG. 2 has, like the first exemplary embodiment, a
substrate 10 with a curved surface 11, a adhesion-promoting layer 1
consisting of Al.sub.2O.sub.3 applied thereto and a following
reflective metal layer 2 consisting of silver. The properties and
the method of production of these layers correspond to the first
exemplary embodiment.
[0047] The second exemplary embodiment differs from the first
exemplary embodiment in the design of the protective layer system
3. As is shown in FIG. 2, the protective layer system 3 has a
sequence of alternating layers 31, 33, these involving
Al.sub.2O.sub.3 layers 31 and Si.sub.3N.sub.4 layers 33. The
protective layer system 3 contains a first layer stack 3A, which is
formed by three layer pairs made up in each case of an
Al.sub.2O.sub.3 layer 31 and a Si.sub.3N.sub.4 layer 33 and also a
final Al.sub.2O.sub.3 layer 31. In the first layer stack 3A, the
Al.sub.2O.sub.3 layers 31 each have a thickness of 5 nm and the
Si.sub.3N.sub.4 layers 33 each have a thickness of 20 nm.
[0048] Furthermore, the protective layer system 3 contains a second
layer stack 3B, which has three layer pairs made up in each case of
a Si.sub.3N.sub.4 layer 33 and an Al.sub.2O.sub.3 layer 31 and also
a final Si.sub.3N.sub.4 layer 33. In the second layer stack 3B, the
Si.sub.3N.sub.4 layers 33 each have a thickness of 5 nm and the
Al.sub.2O.sub.3 layers 31 each have a thickness of 20 nm.
[0049] In total, the protective layer system 3 contains 14
alternating dielectric layers 31, 33, with a multiplicity of
barriers to different contaminations being formed, as in the
previous exemplary embodiment, on account of the multiplicity of
the layer pairs.
[0050] Furthermore, an additional optical function is achieved by
the different layer thicknesses of the individual layers 31, 33 in
the layer stacks 3A, 3B. Since the layer thicknesses of the
individual layers 31, 33 in both layer stacks 3A, 3B in each case
amount to not more than 20 nm, they are small compared to
wavelengths of visible light. Both layer stacks 3A, 3B can
therefore be described approximately by an effective refractive
index given by a refractive index of the individual layers which is
averaged over the respective layer stacks 3A, 3B. This averaged
refractive index is smaller in the first layer stack 3A, in which
the Si.sub.3N.sub.4 layers 33 have a greater thickness than the
Al.sub.2O.sub.3 layers 31, than in the second layer stack 3B, in
which the Si.sub.3N.sub.4 layers 33 have a smaller thickness than
the Al.sub.2O.sub.3 layers 31. Since the Al.sub.2O.sub.3 layers 31
have a greater refractive index than the Si.sub.3N.sub.4 layers,
the effective refractive index of the second layer stack 3B is
greater than the effective refractive index of the first layer
stack 3A.
[0051] The protective layer system 3 therefore forms an optical
interference layer system which is formed by the first layer stack
3A with a relatively low effective refractive index and a second
layer stack 3B with a relatively high effective refractive index.
The multiplicity of the layers 31, 33 and the different layer
thicknesses in the layer stacks 3A, 3B therefore allow for a
further improved setting of the optical properties of the entire
protective layer system 3 compared to a protective layer system
with fewer individual layers.
[0052] The invention is not limited by the description with
reference to the exemplary embodiments. Instead, the invention
encompasses any new feature and also any combination of features,
this including, in particular, any combination of features in the
patent claims, even if this feature or this combination itself is
not explicitly specified in the patent claims or exemplary
embodiments.
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